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Date: Tue, 22 May 2018 15:36:59 -0600
From: Alex Williamson <alex.williamson@redhat.com>
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Subject: Re: [Qemu-devel] [RFC 3/3] acpi-build: allocate mcfg for multiple
 host bridges
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Laszlo Ersek <lersek@redhat.com>, Marcel Apfelbaum <marcel.apfelbaum@gmail.com>, Zihan Yang <whois.zihan.yang@gmail.com>, qemu-devel@nongnu.org, Igor Mammedov <imammedo@redhat.com>, Eric Auger <eauger@redhat.com>, Drew Jones <drjones@redhat.com>, Wei Huang <wei@redhat.com>

On Tue, 22 May 2018 23:58:30 +0300
"Michael S. Tsirkin" <mst@redhat.com> wrote: 
>
> It's not hard to think of a use-case where >256 devices
> are helpful, for example a nested virt scenario where
> each device is passed on to a different nested guest.
>
> But I think the main feature this is needed for is numa modeling.
> Guests seem to assume a numa node per PCI root, ergo we need more PCI
> roots.

But even if we have NUMA affinity per PCI host bridge, a PCI host
bridge does not necessarily imply a new PCIe domain.  Nearly any Intel
multi-socket system proves this.  Don't get me wrong, I'd like to see
PCIe domain support and I'm surprised edk2 is so far from supporting
it, but other than >256 hotpluggable slots, I'm having trouble coming
up with use cases.  Maybe hotpluggable PCI root hierarchies are easier
with separate domains?  Thanks,

Alex